HAZ Microstructural Evolution and Hardness Control in Multi-Layer Weld Overlay on 15CrMo Steel
1. Definition and Fundamental Principles
15CrMo steel is a low-alloy martensitic heat-resistant steel widely used in high-temperature, high-pressure applications including boiler tubes, steam headers, superheater components, and pressure vessels operating in the 450–580°C range. Its nominal composition (approximately 0.15% C, 0.9–1.1% Cr, 0.30–0.60% Mo, with Mn and Si in balance) provides excellent creep resistance and oxidation resistance at elevated temperatures through solid-solution strengthening and fine carbide precipitation (primarily Cr₂₃C₆ and Mo₂C).
Multi-layer weld overlay on 15CrMo steel involves the sequential deposition of multiple weld passes—typically a transition layer, build-up layers, and a final functional cladding layer—to achieve a metallurgically sound joint with desired surface properties (corrosion resistance, wear resistance, or thermal stability). The Heat-Affected Zone (HAZ) represents the region of the base metal that undergoes thermal cycling without melting, experiencing peak temperatures between the Ac₁ (~780°C) and solidus temperature of 15CrMo steel.
The microstructural evolution in the HAZ is governed by the following thermodynamic and kinetic principles:
- Austenitization: Upon heating above Ac₁ (~780°C), the tempered martensite/ferrite-pearlite structure of 15CrMo steel transforms to austenite, dissolving existing carbides and redistributing alloying elements (Cr, Mo) into the austenitic matrix.
- Grain Growth: Extended residence time above Ac₁ promotes austenite grain coarsening, which directly influences the hardness and toughness of the transformed microstructure upon cooling.
- Re-transformation on Cooling: As the weld cools, austenite re-transforms to martensite (if cooling rate exceeds the critical cooling rate), bainite, or a mixed ferrite-bainite structure, depending on the local cooling rate and carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15).
- Tempering and Aging: Subsequent passes act as interpass tempering cycles, partially softening previously deposited martensitic structures and modifying the HAZ microstructure of earlier layers.
2. Category and Business Positioning
This technical entry falls within the company's Weld Overlay Engineering domain, specifically under the TIG/MIG weld overlay technology route. It represents a critical metallurgical knowledge asset that bridges the gap between fundamental materials science and practical overlay fabrication.
In terms of business positioning, this capability addresses:
- Process Qualification Development: Provides the scientific basis for establishing and qualifying Welding Procedure Specifications (WPS) for overlay applications on 15CrMo substrate.
- Quality Assurance: Enables predictive assessment of HAZ properties, allowing proactive control of hardness, residual stress, and crack susceptibility.
- Customer Technical Support: Supports engineering analysis for critical power generation, petrochemical, and nuclear-adjacent applications where 15CrMo components require surface enhancement.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The systematic study of HAZ microstructural evolution during multi-layer overlay on 15CrMo steel serves the following objectives:
- Hardness Mapping and Control: Establish quantitative relationships between interpass temperature, heat input, number of layers, and resulting HAZ hardness profiles. Target hardness for the HAZ should not exceed 250 HV (per NB/T 20316-2011 and ASME Section IX requirements for 15CrMo base metal), with the transition layer hardness gradient managed to minimize mismatch.
- Crack Susceptibility Assessment: Identify critical thermal cycles that promote delayed hydrogen cracking or reheat cracking in the HAZ, particularly in the coarse-grained HAZ (CGHAZ) region.
- Residual Stress Management: Understand how multi-layer deposition patterns influence residual stress states in the HAZ, informing post-weld heat treatment (PWHT) requirements.
- Microstructural Integrity: Ensure that repeated thermal cycles do not produce unacceptable grain coarsening or brittle phase formation in the 15CrMo HAZ.
3.2 Value Contribution
- Reduces qualification testing cycles by providing predictive models for HAZ behavior
- Minimizes rework rates through optimized interpass temperature control
- Enables delivery of overlay-clad components meeting stringent code requirements (NB/T 20316, ASME Section IX, API 579)
- Provides technical documentation supporting customer audits and regulatory inspections
4. Key Process and Implementation Points
4.1 Multi-Layer Overlay Architecture
A typical multi-layer overlay on 15CrMo steel comprises the following configuration:
| Layer | Function | Typical Material | Thickness per Pass | Key Requirement |
|---|---|---|---|---|
| Sub-preparation | Surface cleaning and preheat | — | — | Preheat to 200–250°C; removal of scale and contaminants |
| Transition Layer (Layer 1) | Dilution buffer; strain accommodation | 309L / 310L / 2205 duplex | 3–5 mm | Low carbon; high Cr-Ni for ductility |
| Build-up Layers (Layer 2–n-1) | Dimensional build; property gradient | 309L / 316L / 2205 | 4–6 mm per layer | Controlled interpass temperature; consistent bead geometry |
| Functional Cladding Layer (Layer n) | Surface protection (corrosion/wear/thermal) | 316L / 625 / Stellite 6 / 2507 | 2–4 mm | Final composition; minimal dilution from underlying layers |
4.2 Critical Process Parameters
| Parameter | Recommended Range | Effect on HAZ | Control Method |
|---|---|---|---|
| Preheat Temperature | 200–250°C | Reduces cooling rate; minimizes martensite formation; reduces residual stress | Infrared pyrometer; thermocouple monitoring |
| Interpass Temperature | 150–250°C (max 300°C) | Higher interpass T reduces HAZ hardness but risks grain growth if excessive | Real-time surface temperature monitoring between passes |
| Heat Input | 1.5–4.0 kJ/mm (TIG); 10–30 kJ/mm (MIG) | Higher heat input increases HAZ width; promotes grain coarsening | Controlled travel speed and wire feed rate |
| Welding Current (TIG) | 80–180 A | Affects penetration depth and HAZ thermal profile | WPS-defined parameters with ±10% tolerance |
| Shielding Gas | 100% Ar (TIG); Ar/CO₂ 80/20 or Ar/He 75/25 (MIG) | Prevents oxidation; ensures clean weld metal chemistry | Flow rate control (8–12 L/min TIG; 15–25 L/min MIG) |
| Number of Layers | 3–8 layers typical | More layers = more thermal cycles = cumulative HAZ grain growth | WPS-defined layer count; NDE after each layer |
4.3 HAZ Microstructural Zones and Their Behavior
The HAZ in 15CrMo steel during overlay welding can be subdivided into distinct sub-zones, each exhibiting different microstructural responses:
| HAZ Sub-zone | Peak Temperature | Microstructural Change | Typical Hardness (HV) | Risk Factor |
|---|---|---|---|---|
| CGHAZ (Coarse-Grained HAZ) | 1200–1400°C | Austenite grain coarsening; rapid transformation to martensite/bainite on cooling | 280–380 HV (as-welded) | High crack susceptibility; toughness degradation |
| IGHAZ (Intermediate-Grained HAZ) | 1000–1200°C | Moderate grain growth; mixed ferrite-bainite on cooling | 220–290 HV (as-welded) | Moderate; sensitive to cooling rate |
| PAZ (Partially Recrystallized Zone) | 900–1000°C | Partial austenitization; heterogeneous microstructure | 200–250 HV | Low; property discontinuities possible |
| Tempered Zone | 600–780°C (Ac₁) | Tempering of existing martensite; carbide coarsening | 180–220 HV | Softening; potential for undermatch |
4.4 Hardness Control Strategy
The following systematic approach is employed to control HAZ hardness within acceptable limits:
- Preheat Optimization: Maintain preheat at 220–250°C to ensure cooling rates below the critical rate for martensite formation (typically < 15°C/s for 15CrMo steel with CE ≈ 0.45–0.55).
- Interpass Temperature Management: Keep interpass temperature between 150–250°C. This provides beneficial tempering of the previous pass while avoiding excessive grain growth. Exceeding 300°C risks sensitization in austenitic transition layers.
- Heat Input Limitation: For TIG overlay, limit heat input to 3.0 kJ/mm maximum. For MIG overlay, control wire feed rate and travel speed to maintain heat input below 25 kJ/mm.
- Layer Sequencing: Alternate welding directions between layers to distribute thermal stress and minimize cumulative HAZ damage in any single direction.
- Post-Weld Heat Treatment: Apply PWHT at 700–720°C for a minimum of 1 hour per 25 mm thickness (per NB/T 20316-2011) to relieve residual stresses and normalize HAZ microstructure.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Relevant Requirement |
|---|---|---|
| NB/T 20316-2011 | Power plant welding procedure qualification and welder performance qualification | WPS qualification; PWHT requirements; hardness testing limits |
| ASME Section IX | Qualification rules for welding, brazing, and fuse bonding | Essential variables for overlay; qualification test requirements |
| ASME Section VIII Div. 2 | Rules for construction of pressure vessels (alternative rules) | Clad pressure vessel requirements; weld overlay acceptance |
| GB/T 985.1-2008 | Welding procedure specification content | WPS documentation requirements |
| GB/T 3375-2017 | Basic terms in welding, brazing and cutting | Terminology and definitions |
| ASTM A204 | Standard specification for chromium-molybdenum steel plate | 15CrMo (SAE 15Mo) base metal chemistry and mechanical properties |
| NACE MR0175/ISO 15156 | Sulfide-resistant materials for H₂S-containing environments | Hardness limits for overlay materials in sour service |
| API 579-1/ASME FFS-1 | Fitness-for-service assessment | Residual stress assessment; hardness-based screening |
| GB/T 6394-2017 | Metallographic examination of metals | Microstructural examination procedures |
| ISO 6508-1:2016 | Hardness testing of metals — Vickers hardness test | Hardness measurement methodology |
5.2 Acceptance Criteria
- HAZ Hardness: Maximum 250 HV after PWHT (per NB/T 20316-2011); maximum 22 HRC equivalent (per NACE MR0175/ISO 15156 for sour service applications).
- Hardness Gradient: Maximum change of 50 HV per mm from the fusion line into the base metal HAZ.
- Weld Metal Hardness: Transition layer: 180–260 HV; Functional cladding: per material specification (e.g., 316L: 180–250 HV; Stellite 6: 300–400 HV).
- Microstructural Integrity: No untempered martensite in HAZ after PWHT; no intergranular cracking; no excessive grain coarsening (austenite grain size ≤ ASTM No. 4 after simulated thermal cycle).
- NDE Acceptance: 100% surface NDE (MT/PT); 100% UT thickness measurement; 100% visual inspection per ASME Section V Article 1.
- Penetration: Minimum 0.5 mm penetration into 15CrMo base metal for first transition layer (per ASME Section IX QW-451 for overlay welding).
6. Common Risks and Controls
6.1 Risk Identification and Mitigation Matrix
| Risk | Mechanism | Detection Method | Preventive Control | Corrective Action |
|---|---|---|---|---|
| Delayed hydrogen cracking in HAZ | High CE of 15CrMo; martensite formation; hydrogen diffusion | MT after 24–48 hours; DWUT | Preheat ≥ 200°C; low-hydrogen shielding; limit hydrogen in consumables | Full PWHT at 620°C/2h; re-inspection |
| Reheat cracking in CGHAZ | Stress concentration; precipitate-free zone; sulfur/phosphor segregation | MT after PWHT; macroscopic examination | Limit S < 0.015%; P < 0.025%; controlled PWHT heating rate ≤ 200°C/h | Rework with preheat; PWHT requalification |
| Excessive HAZ hardness (>250 HV) | High cooling rate; insufficient preheat; high heat input variation | Hardness survey per ISO 6508-1:2016 | Strict preheat and interpass temperature control; heat input monitoring | Local PWHT; overlay layer removal and re-deposition |
| Grain coarsening in HAZ | Multiple thermal cycles; high interpass temperature; excessive heat input | Microstructural examination per GB/T 6394-2017 | Limit number of layers; control interpass T ≤ 250°C | PWHT normalization; additional overlay layer to mask |
| Undermatch (soft HAZ) | Excessive tempering; low-carbon transition layer dilution | Hardness survey; tensile testing | Control interpass T; adequate preheat; proper transition layer selection | Rebuild overlay; PWHT optimization |
| Porosity and gas inclusions | Moisture in consumables; inadequate shielding; contamination | UT; RT; visual inspection | Dry consumables; gas flow verification; surface cleaning | Defect removal; re-weld per WPS |
6.2 Residual Stress Management
Multi-layer overlay on 15CrMo steel generates complex residual stress fields due to:
- Thermal contraction of deposited weld metal upon cooling
- Phase transformation stresses during martensite formation
- Constrained deformation between layers with different thermal expansion coefficients
Typical residual stress levels in the HAZ can reach 300–450 MPa (tensile) in the as-welded condition. PWHT at 700–720°C for 1 hour per 25 mm thickness reduces residual stresses to below 100 MPa, satisfying the requirements of NB/T 20316-2011 and ASME Section VIII.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
This technical entry is most directly applicable to the TIG/MIG weld overlay route, where HAZ microstructural control is paramount. Key applications include:
- Power Generation: Overlay of corrosion-resistant cladding (316L, 625, or 2205) on 15CrMo steam pipes, superheater tubes, and economizer headers in subcritical and supercritical boilers. The HAZ hardness control knowledge ensures that the overlay does not compromise the structural integrity of the 15CrMo substrate.
- Petrochemical: Wear-resistant overlay (Stellite 6, 6/6) on 15CrMo valve bodies, flange faces, and pump shafts operating in abrasive slurry service. Hardness matching between overlay and HAZ prevents undermatch and premature failure.
- Repair and Extension of Life: Rebuilding of worn 15CrMo components (rotary kiln liners, cement mill trunnions) with multi-layer overlay. Understanding HAZ evolution allows optimization of the number of layers and interpass conditions for maximum service life.
- Hydrogen Service: Overlay of low-hydrogen-permeability materials on 15CrMo pressure vessels in hydrogen compression applications. Strict hardness control (< 22 HRC) per NACE MR0175/ISO 15156 prevents hydrogen-induced cracking.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding does not involve welding heat input, the HAZ microstructural knowledge of 15CrMo steel contributes indirectly through:
- Base Metal Characterization: Understanding the base 15CrMo microstructure (tempered martensite with fine carbide dispersion) enables prediction of bonding interface behavior under high-strain-rate deformation.
- Post-Bonding Heat Treatment: When hydraulic explosive bonded clad plates require subsequent welding (e.g., edge welding, drilling, or machining), the HAZ knowledge informs preheat and PWHT requirements to maintain the bond integrity.
- Composite Material Design: For hybrid configurations where explosively bonded cladding is followed by weld overlay on the cladding surface, HAZ hardness control ensures that the weld overlay does not induce cracking at the explosive bond interface.
7.3 Explosion Welding Route
Explosion welding of 15CrMo steel with dissimilar cladding materials (e.g., 316L, 2205, copper, or nickel alloys) produces a mechanically bonded interface without a weld HAZ in the conventional sense. However, the technical knowledge contributes in the following ways:
- Interface Metallurgy Correlation: The understanding of 15CrMo phase transformations during rapid thermal cycling (analogous to the explosive welding collision event) informs predictions about intermetallic compound formation at the bond interface.
- Post-Welding Operations: Explo-sion welded clad plates of 15CrMo frequently require subsequent weld overlay for dimensional finishing, edge repair, or functional surface addition. The HAZ evolution knowledge directly governs the WPS for these secondary welding operations.
- Qualification Integration: For nuclear-adjacent or pressure vessel applications (per NB/T 20316-2011), explosion-welded 15CrMo clad plates must demonstrate that any subsequent welding operations do not degrade the base metal properties. HAZ hardness and microstructural data from this study provide the technical basis for such qualification.
- Hybrid Cladding Solutions: In complex geometries where explosion welding alone is insufficient (e.g., small-diameter pipes, complex curvatures), a hybrid approach combines explosion welding for bulk cladding with TIG weld overlay for finishing. HAZ control knowledge ensures metallurgical compatibility throughout the composite structure.
8. Qualification Building and Customer Value
8.1 Qualification Building
This technical capability directly supports the company's qualification framework through:
- WPS Development: Provides the scientific basis for establishing qualified WPS for multi-layer overlay on 15CrMo steel, reducing the number of trial coupons and accelerating qualification timelines.
- WPQ Support: Informs welder performance qualification requirements by defining critical variables (preheat, interpass temperature, heat input) that must be controlled within specified ranges.
- Material Qualification: Supports the qualification of transition layer materials (309L, 310L, 2205) through demonstrated understanding of dilution effects and HAZ property interactions.
- Code Compliance: Ensures deliverables meet NB/T 20316-2011, ASME Section IX, and ASME Section VIII Div. 2 requirements for overlay welding on chromium-molybdenum steels.
8.2 Customer Value Proposition
- Reduced Lifecycle Cost: Optimized HAZ properties extend component service life by 30–50% compared to unoptimized overlay procedures, reducing replacement frequency and unplanned shutdown costs.
- Accelerated Delivery: Predictive metallurgical models reduce qualification testing cycles by 40–60%, enabling faster project timelines for customers in power generation and petrochemical sectors.
- Risk Mitigation: Proactive identification and control of HAZ-related failure modes (cracking, undermatch, excessive hardness) reduces warranty claims and field failures.
- Technical Authority: Demonstrated deep understanding of 15CrMo overlay metallurgy positions the company as a preferred supplier for critical, high-consequence applications where failure is not an option.
- Regulatory Confidence: Comprehensive metallurgical documentation and controlled HAZ properties facilitate smoother regulatory inspections and customer audits, reducing project approval delays.
9. Conclusion
The systematic study of HAZ microstructural evolution during multi-layer weld overlay on 15CrMo steel represents a cornerstone technical capability for Cladding Technology Shanxi Co., Ltd. By establishing quantitative relationships between process parameters (preheat, interpass temperature, heat input, layer count) and resulting HAZ properties (hardness, microstructure, residual stress), the company enables reliable, code-compliant delivery of overlay-clad components across power generation, petrochemical, and industrial applications.
This knowledge asset integrates seamlessly across all three technology routes—TIG/MIG weld overlay (direct application), hydraulic explosive bonding (post-bonding welding operations), and explosion welding (secondary finishing welds)—providing a unified metallurgical framework for composite material fabrication. The resulting qualification strength, quality assurance capability, and technical authority deliver measurable value to customers through extended service life, reduced downtime risk, and accelerated project execution.